Hello everyone. Currently working on my first bathroom reno, have watched quite a bit of youtube, mostly Sal's videos, and read a lot on this site. Taking it slow and trying to do everything right.

I'm currently at the stage where I have the backer board up in the shower stall coated in red guard. I did a 3 piece drain with pre-slope, liner, and also red guarded the shower pan for a "second liner" like Sal does. For the floor, I'm planning to use a decoupling membrane for heating wire like ditra-heat or similar (whatever is the best price).

The instructions from these two products, ditra-heat and red guard, leads to my confusion about which thinset to use. Schluter says to use a modified thinset between the plywood floor and the membrane, as the wood is breathable and will let the modified thinset dry out, whereas, according to their instructions, you should use an unmodified thinset between the membrane and the tile, as they are two impermeable surfaces, that will allow the unmodified thinset to stay moist and cure properly. This methodology however, does not seam to carry over to the red guard product, where I would also be putting thinset between two impermeable layers, yet red guard recommends a modified thinset.

I also have the added complication of using "river rock" for my shower floor, which will result in large grout lines and thus lots of "breath-ability", so likely the modified would be best in this situation? I'm also looking for recommendations about trowel size for unshaved, but quite flat "river rocks", naturally collected, will be single laid, no mesh. They're about 2-3 inches in diameter. Though a bit tedious, I'm thinking it would probably be prudent to back butter each little one, since they're not shaved completely flat.

And I just wanna say thanks for everyone who's contributed to the wealth of information on this forum. Wouldn't be possible for folks like me to take something like this on without access to the information to figure it out. At one point I was about to use my extra backer board on the floor before putting down the decoupling membrane, but a quick search here saved me from a big mistake and a lot of headache! Put down another layer of plywood instead.

If you're constructing a traditional shower receptor you do not want to apply any kind of direct bonded waterproofing membrane to the top mud surface. It's a very bad idea regardless what Sal or anyone else might tell you.

Use one method of receptor waterproofing, use it correctly, and it will work just fine.

The mortar you use for bonding to the plywood subfloor must be a modified thinset mortar meeting ANSI A118.11. Must be. It will have those numbers on the bag.

I would recommend you use the same mortar for installing the "river rocks" to your shower floor. You'll be bonding to the deck mud there and not RedGard or any other membrane. I'd recommend you increase the slope of your shower floor to something closer to 1/2" per horizontal foot to provide good drainage with those rocks.

Same principle in your substrate for your bathroom floor. Pick one type of substrate and install it properly and it will work just fine.

If the pebbles (rocks) vary in size then you'll definitely have to do some buttering up. I would probably set the drain height to one of the thicker stones and butter the others up to it. Buttering will be important to avoid low areas. Using a thinset made for large format tiles will help hold up the buttered stones.

I have modified ANSI A118.11 thinset I was planning to use for everything, but the Schluter instructions for ditra-heat are telling me to use unmodified between the ditra and the tiles because both layers are impermeable. The same applies to the red guard membrane on the shower walls, yet red guard recommends a modified thinset.

If the thought conveyed by Schluter that unmodified thinset should be used between two impermeable layers holds true, then why does red guard not seem to agree, and what should I do?

Completely separate from this, why is it a bad idea to waterproof the mud surface? Sal seems like a very well respected professional, so I didn't hesitate to follow his build procedure. I'm not allowed to post links yet, but I'm referring to Sal DiBlasi, he has a large youtube channel.

Thanks for the tip on more slope with the river rocks. Luckily I thought of that one myself and I'm right around 1/2" drop per foot. from the corners, slightly more from the edges.

Jesse, every manufacturer of direct bonded waterproofing membranes except Schluter recommends or requires the use of a modified thinset mortar for use on either side of their membrane.

Sal is a well respected member of this forum, but he's wrong about using two membranes to construct a shower receptor because there is no corrrect method of connecting the second membrane to the drain per all the manufacturers' instructions. That's the membrane manufacturers and the drain manufacturers.

Moisture will eventually get through the top membrane into the top mud bed and will have no way to evaporate. What harm will it do? I dunno, but I don't want it there in my shower floor.

My understanding is you want to pick an approved shower receptor system and stick with it, do not create a hybrid. If you are creating a traditional shower receptor with a pan liner, mud bed and weep hole drain then you need to allow the system to function: water in water out. If you redguard the cbu then it creates a situation when the moisture can wick up the walls from the pan and be a problem. If you are creating a bonded tile shower system like Schluter Kerdi then you use a Kerdi drain so the moisture never enters the walls. Look at the TCNA Handbook for approved methods. I am sure there is an approve drain to use if you regard walls and receptor.
I am also not sure I would use Ditra heat with river rock. Again it is a system that is engineered to allow heat to escape and with uneven thicknesses it may cause heat to build up and cause failure. I would check with Schluter.
I am sure the pros here have a better description of how these things work.

__________________
Jim

The definition of insanity is doing the same thing over and over and expecting different results.

Sorry if I wasn't clear, the ditra-heat is for the bathroom floor, which I haven't bought tile for yet, but will likely be porcelain. Only the shower floor will be river rocks.

At this point, I have already installed the CBU slightly embedded in the mud bed like Sal does, but I did take the extra step of red guarding 1" up on the inside, outside, and bottom edge of the CBU. I know.. no vapour barrier on both sides, but it's only 1" up on the inside, and it will stop any wicking into the wall from the mud bed, if it were to ever get wet, the water would only be able to leave through the weep holes.

I realize that creating any type of hybrid might be offensive to the "rules", but common sense and listening to pros has me not trusting a bead of silicone to do a better job of stopping water from wicking into the CBU. I think at this point I will continue with Sal's method, but in the future I would likely use a bonded drain system.

Back to my main question though, regarding the thinset, seems there is a discrepancy with the "rules" (which is why some hybridding might not always be a bad idea), what would you use CX, modified or unmodified between the ditra and tile? If I'm going to use modified between the red guard and tile, then does it really make sense to switch to unmodified between the ditra and tile? Even if those are the "rules"...

Jesse, I cannot tell you what you should or shouldn't do. I can only say that I have successfully used Custom's highly modified Flexbond (an excellent product, by the way) between my Ditra and LFT porcelain floor tiles. I would not hesitate to use any polymer-modified quality thinset with any Schluter product.

I would hesitate, however, to use latex-modified thinsets with waterproofing membranes as they apparently need to literally dry to form their final strength. I would also hesitate to build a hybrid systems as I wouldn't want to deal with possible failure (however unlikely it might be). The possibility of trapping moisture that CX raises in his response would deter me from attempting such a system. Not enough benefits to outweigh the possible downsides.

Over the years we've used a truck load of Ditra and have never used unmodified thinset under it or over it. I don't have Schluter's warranty but I feel I won't need it if I use modified and give it a little extra time to dry out.

Jessie, keep in mind that in a shower water should only go one direction, that's down the drain. But, eventually hair and soap will slow the water flow down and water will likely slow down enough to back up the weep holes and into the mud bed. In my opinion, when it comes to membranes, more is not better.

THere aren't too many latex based modified thinsets out there anymore, but you definitely don't want one of those between your tile and a waterproof membrane. Schluter's issue is that most people don't know enough to be able to discern the actual type of modifier in their thinset and they've found that for most cases, an unmodified works just fine. If you want a modified and their warranty, the do offer a modified they know will work. There are certainly others that will as well...so, it's not a modifier per se, it's certain types of modifiers that are the issue. Schluter is very conservative and would like for your end result to work. They know an unmodified will work, some modified ones won't unless you take some precautions that most people won't abide by.

Wolfgang, to me, both situations seem the same, you are putting the thinset between an impermeable membrane and the tile, where it can't breathe very well to dry out. Schluter recommends unmodified (or their own brand of modified only) for this, while red guard recommends modified. Unless I am misunderstanding, you are suggesting the opposite.

I did some more research. Both Aqua Defense and HydroBan recommend modified thinset, so the topical membranes are in agreement.

The prodesso heat membrane however says to use unmodified between the membrane and tile, just as ditra does, while drexma-t doesn't indicate either way, and warmup membranes say you can use whatever you want, modified, unmodified, or SLC for small format tiles or other floor coverings.

There's definitely some conflicting information here. It's hard to be confident making a decision as a newbie. Maybe I'm over thinking things though, and should just use the custom products versa bond that I already have (not quite as good as flex bond by the looks of it, but it's still ANSI A118.11).

As for the second membrane in the shower stall, you've all got me reconsidering now. I thought there was no going back, as I've already set the CBU into the mud bed as opposed to leaving a 1/4" gap to fill with silicone. I just read the blurb on this website about wicking up the walls though, where the year long test was done, and now it seems like it's probably not an issue and that was a good way to secure the lower portion of the CBU anyway, since you can't screw down that low. I also took the extra precaution of waterproofing the entire bottom inch of the CBU, and it's only embedded about 1/2" into the mud bed, so that should protect against wicking anyway. Should I really now abandon the teachings of Sal? Maybe so.. keep convincing me.

Thanks for that information Jim. You posted while I was writing my last reply. That definitely sheds some light on what's going on here. I'm planning to use versa-bond from custom products. It says polymer modified, but then again, latex is a polymer. How do I figure out if this mortar is acceptable? What is thinset now usually modified with if not latex polymers?

Latex is a misnomer. It is a descriptive term to describe a milky substance. But true latex is not used in thinset or paint. They use pva or acrylic polymers. The purpose of polymers is to slow the drying of the cement so it develops a stronger bond in a thin application. In a shower if you use a polymer modified thinset and it does not properly cure it can leach the "latex" and the cement won't cure properly. Any chemists can jump in and correct if I have it wrong.

__________________
Jim

The definition of insanity is doing the same thing over and over and expecting different results.

Jesse, I am not a chemist but my understanding is that the "polymer" designated thinsets have some sort of man-made polymer in them. I think you're technically correct that latex is a polymer but I have seen products marked as "latex modified". They tend to be the more old-fashioned products. I do not know for sure.
Only certain modified thinsets require the air drying. Based on my cursory readings, these seem to be the latex ones.
Having been curious, I tested the Flexbond, sealing some thinset in a plastic container where it had no ability to dry. It cured rock hard in 48 hours. There was no indication that it would fail.
You can't really go wrong with Versabond. It's good stuff and more economical than the Flexbond. I like the Versabond LFT.